1. Field of the Invention
This invention relates to a form of herpesvirus particles suitable for use in vaccination, the preparation of such particles, and vaccines containing them.
2. Description of the Related Art
Herpes simplex virus type 1 (HSV-1) light particles (L-particles) are non-infectious virus-related particles, produced in approximately equal numbers with virions throughout the virus replication cycle in BHK cells (Szilagyi and Cunningham, 1991, J. Gen. Virol. 72, 661-668; Rixon et al., 1992, J. Gen. Virol. 73, 277-284). Similar L-particles have been isolated from pseudorabies virus (PRV); equine herpesvirus type 1 (EHV-1) (McLauchlan and Rixon, 1992, J. Gen. Virol. 73, 269-276); bovine herpes virus type 1 (BHV-1); varicella-zoster virus (VZV) (Dargan and Subak-Sharpe, unpublished) and HSV-2 (MacLean, unpublished). Because of the non-infectious nature of L-particles they have great potential as candidate vaccine materials (PCT Patent Application Publication No. 92/19748).
Comparative analysis of the protein composition of HSV-1, PRV and EHV-1 L-particles and virions show that most or all the virion tegument and envelope proteins are present in L-particles, but the nucleocapsid proteins are not present. Five phosphoproteins not detectable in HSV-1 virions are associated with HSV-1 L-particles (Szilagyi and Cunningham, 1991, J. Gen. Virol. 72, 661-668; McLauchlan and Rixon, 1992, J. Gen. Virol. 73, 269-276).
HSV-1 L-particles have been shown to be as efficient as virions in supplying functional tegument proteins to target cells. Thus, L-particles are biologically competent and have the potential to participate in the early stages of HSV-1 infections (McLauchlan et al., 1992, Virology, 190, 689-688).
Using an HSV-1 ts mutant (ts1201) (Preston et al., 1983, J. Virol. 45, 1056-1064) having a mutation in gene UL26, Rixon et al., 1992, J. Gen. Virol. 73, 277-284, demonstrated that L-particles were generated independently of virion maturation. Under non-permissive conditions ts1201 failed to make infectious virions but produced L-particles which were identical to typical wild-type virus L-particles in appearance and protein composition. Although viral DNA is synthesised normally in cells infected with ts1201 under their non-permissive conditions, viral DNA packaging into virions is blocked (Preston et al., 1983, J. Virol. 45, 1056-1064).
Although L-particles can be prepared to be substantially free of infectious virions, a typical preparation of HSV-1 L-particles containing a ratio of from 3.times.10.sup.3 :1 to 1.times.10.sup.4 :1 L-particles: infectious virions, there is a problem to improve on this ratio. Further, although the L-particles lack a capsid and the DNA within it, L-particle preparations still contain some viral DNA present in contaminating virions and/or possibly in the form of free nascent viral DNA. It would be advantageous to reduce the amount of such DNA present in the L-particle preparations, in order more easily to convince regulatory authorities of the safety of a vaccine containing them.
Additional prior art, the relevance of which becomes clear only in the context of the present invention, is referred to below after "Summary of the invention".